1. Field of the Invention
The present invention relates to a drive circuit for driving a light emitting element such as a laser diode used in optical communications etc.
2. Description of the Related Art
FIG. 1 is a circuit diagram of an example of the configuration of a drive circuit for a laser diode used as a light emitting element of the prior art.
This laser diode use drive circuit 10 is constituted by an input circuit 11, a level shift circuit 12, a differential output circuit 13, and a current setting circuit 14. Further, LD indicates a laser diode.
The input circuit 11 is constituted by npn type transistors Q111 and Q112, resistors R111 and R112, and a current source I111.
Emitters of the transistors Q111 and Q112 are connected to each other, and a connection point of these emitters is connected to the current source I111.
A base of the transistor Q111 is connected to an input terminal TD of a drive signal D, and a collector is connected to a supply line of a power supply voltage V.sub.cc via the resistor R111.
The base of the transistor Q112 is connected to an input terminal TDB of a drive signal DB taking a complementary level of the drive signal D, and the collector is connected to the supply line of the power supply voltage V.sub.cc via the resistor R112.
The level shift circuit 12 is constituted by npn type transistors Q121, Q122, Q123, and Q124 and current sources I121 and I122.
The base of the transistor Q121 is connected to the collector of the transistor Q111 of the input circuit 11, the collector is connected to the supply line of the power supply voltage V.sub.cc, and the emitter is connected to the collector and base of the transistor Q123. Namely, the transistor Q123 is diode-connected, and the emitter thereof is connected to the current source I121.
The base of the transistor Q122 is connected to the collector of the transistor Q112 of the input circuit 11, the collector is connected to the supply line of the power supply voltage V.sub.cc, and the emitter is connected to the collector and base of the transistor Q124. Namely, the transistor Q124 is diode-connected, and the emitter thereof is connected to the current source I122.
The differential output circuit 13 is constituted by npn type transistors Q131, Q132, and Q133 and a resistor R131.
Emitters of the transistors Q131 and Q132 are connected to each other, and the connection point of these emitters is connected to the collector of the transistor Q133 serving as the current source.
The base of the transistor Q131 is connected to the emitter of the transistor Q124 of the level shift circuit 12, and the collector is connected to a (cathode of the) laser diode LD via a connection terminal T131.
The base of the transistor Q132 is connected to the emitter of the transistor Q123 of the level shift circuit 12, and the collector is connected to the supply line of the power supply voltage V.sub.cc via a connection terminal T132.
The emitter of the transistor Q133 is grounded via the resistor R131.
The current setting circuit 14 is constituted by an npn type transistor Q141, a resistor R141, and an externally attached current source I141.
The base of the transistor Q141 is connected to the base of the transistor Q133 of the differential output circuit 13 and, at the same time, connected to the collector of the self, and the collector is connected to the current source I141 via a connection terminal T141. The emitter of the transistor Q141 is grounded via the resistor R141.
A so-called current mirror circuit is constituted by the transistor Q141 and the resistor R141 of this current setting circuit 14 and the transistor Q133 and the resistor R131 of the differential output circuit 13.
In this case, for example the transistor size (emitter size) of the transistor Q133 of the differential output circuit 13 is set to n times the transistor size of the transistor Q141 of the current setting circuit 14.
Further, when the resistance value of the resistor R141 of the current setting circuit 14 is defined as R. the resistance value of the resistor R131 of the differential output circuit 13 is set to R/n.
By this, an n x Iset modulation current will be supplied to the differential output circuit 13.
In a state where the modulation current of n times the current Iset by the current source I141 of the current setting circuit 14 is supplied to the differential output circuit 13 in this way, the differential circuit comprising the transistors Q131 and Q132 of the differential output circuit 13 is switched and driven in accordance with the input level of the drive signal D so as to drive the laser diode LD to emit light.
Specifically, when drive signals D and DB are supplied to the input circuit 11 at a high level (H) and a low level (L), respectively, the current flowing in the transistor Q111 side is increased (transistor Q111 becomes ON state) and the current flowing in the transistor Q112 side is reduced (transistor Q112 becomes OFF state).
As a result, the collector current of the transistor Q111 is increased, the transistor Q121 of the level shift circuit 12 becomes the OFF state, and there is a level shift (voltage drop here) at the transistor Q123 serving as the diode. This is supplied to the base of the transistor Q132 of the differential output circuit 13.
At this time, the transistor Q124 of the level shift circuit 12 becomes a high state. This is supplied to the base of the transistor Q131 of the differential output circuit 13.
Accordingly, the current flowing in the transistor Q131 of the differential output circuit 13 is increased (transistor Q131 becomes ON state) and the current flowing in the transistor Q132 side is reduced (transistor Q132 becomes OFF state).
As a result, the laser diode LD emits light.
On the other hand, when drive signals D and DB are supplied to the input circuit 11 at a low level (L) and a high level (H), respectively, the current flowing in the transistor Q112 side is increased (transistor Q112 becomes ON state), and the current flowing in the transistor Q111 side is reduced (transistor Q111 becomes OFF state).
As a result, the collector current of the transistor Q112 is increased, the transistor Q122 of the level shift circuit 12 becomes the OFF state, and the input signal subjected to the amplification action is further shifted in level (voltage drop here) at the transistor Q124 serving as the diode and supplied to the base of the transistor Q131 of the differential output circuit 13.
At this time, the transistor Q123 of the level shift circuit 12 becomes a high state. This is supplied to the base of the transistor Q132 of the differential output circuit 13.
Accordingly, the current flowing in the transistor Q132 of the differential output circuit 13 is increased (transistor Q132 becomes ON state), and the current flowing in the transistor Q131 side is reduced (transistor Q131 becomes OFF state).
As a result, the laser diode LD does not emit light.
As described above, by switching the differential pair of transistors Q131 and Q132 of the differential output circuit 13 in accordance with the input level of the drive signals D and DB, the emission of light by the laser diode LD is controlled.
Along with the demands for reduction in the power consumption in recent years, however, a reduction of the power supply voltage (for example 3.3V) is also being sought for the drive circuit for light emitting elements used in optical communications, etc.
The problem when trying to realize this is the characteristic of the forward direction bias voltage (Vf) at the time of driving possessed by a laser diode or other light emitting element.
This forward direction bias voltage Vf must be as much as 1.8 to 2.0V for obtaining the desired light power over the entire temperature range, but it is difficult to satisfy this characteristic by a 3.3V power source in the drive circuit of the prior art mentioned above.
Namely, as the circuit for setting the modulation current in the drive circuit 10 of the prior art, a current mirror circuit is used as mentioned above, but the voltage (V.sub.x) required for the constant current source comprising this current mirror circuit must be as low as about 1.0V (at the time of a low temperature and a large current). The Vf (V.sub.LD) of the drive transistor (Q131) can only be held at only 1.0V at the most.
Accordingly, in the drive circuit 10 of the prior art, it is impossible to obtain 2.0V for the voltage (V.sub.LD) to be supplied to the light emitting element when considering the temperature characteristic so far as a current mirror circuit is used.